Control system for stopping and starting motor



June 26, 1956 A. c. HALTER CONTROL SYSTEM FOR STOPPING AND STARTING MOTOR 2 Sheets-Sheet 1 Filed Nov. 18, 1953 June 26, 1956 A. c. HALTER CONTROL SYSTEM FOR STOPPING AND STARTING MOTOR 2 Sheets-Sheet 2 Filed Nov. 18, 1953 United States Patent CONTROL SYSTEM FOR STOPPING AND STARTING MOTOR Allan C. Halter, Milwaukee, Wis., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis.

Application November 18, 1953, Serial No. 392,810 7 Claims. (Cl. 318-143) This invention relates in general to control systems for starting and stopping motors and in particular to systems for starting and stopping rolling mill motors which are driven by a main generator and controlled by booster generators.

In systems of the above type, the speeds of the motors may be controlled by field control of the generators, and by such control the motors can be urged toward zero speed. The residual magnetism of the generators, however, even at regulation for Zero speed, causes the generators to develop some voltage output which drives the motors causing slow turning or creeping of the motors.

In prior art systems of this type, creeping is overcome and the mill motors are completely stopped by opening main line switches to interrupt the flow of current to the armatures of the motors. These systems have the disadvantage that arcing occurs at the switches and that therefore the switches require maintenance and repair. Another disadvantage of such systems is that the motors are stopped and started jerkily because all the switches are not opened and closed exactly simultaneously.

It has been discovered that these disadvantages are overcome by providing for the booster generators and the main generator, regulating means responsive to either a first control voltage or a second control voltage (or, to a first or second current), and by providing switching means actuated by the mill speed selector to adapt the regulating means to respond to either of these control voltages (or currents) so that the mill motors may be completely stopped and again started without opening the main line switches.

It is therefore an object of this invention to provide a new and improved control system for a rolling mill which completely stops and again starts the motors of the mill smoothly and without opening the main line motor switches.

Another object of this invention is to provide a control system for smoothly starting and stopping a motor.

A further object of this invention is to provide a control system for starting and stopping a motor without opening a switch in the circuit of the motor armature.

Objects and advantages other than those above set forth will be apparent from the following description when read in conjunction with the drawing, in which:

Fig. 1 shows the invention embodied in a control system for a rolling mill;

Fig. 2 shows, in more detail, a magnetic amplifier shown in Fig. 1;

Fig. 3 shows, in more detail, another magnetic amplifier shown in Fig. 1;

Fig. 4 shows, in more detail, a still ditferent magnetic amplifier shown in Fig. 1; and

Fig. 5 shows, in more detail, still another magnetic amplifier shown in Fig. 1.

In the drawings, where an underlined reference numeral appears in proximity with a plurality of lower case reference letters, the numeral indicates a means comprising a plurality of elements and the elements are indicated by as the lower case reference letters. In the specification, these elements are identified by the reference numeral accompanied by the reference letter.

Referring to Fig. 1, rolls 10 of the first stand of the mill and rolls 11 of the second stand of the mill are adapted to move a strip 12 in the arrow direction of the mill. Delivery tension is provided by tension rolls 13 and 14.

The rolls 10, 11, 13 and 14 are driven by motors 15, 16, 1'7 and 18 respectively; these motors have armatures 15a, 16a, 17a and 18a, series field windings 15b, 16b, 17b and 18b, and separately excited field windings 15c, 16c, 17c and 180. The separately excited field windings are energized from direct current control bus 20 and their energization may be regulated by rheostats 21, 22, 23 and 24, respectively.

A main generator 19 having an armature winding 19a, a series field winding 1%, and a separately excited field winding 19c, supplies current to the motor armatures through main line switches 5, 6, '7 and 8. Booster generators 25, 2'7 and 28 have their armatures 25a, 27a and 28a connected in series with motor armatures 15a, 17a and 18a and connected between the main generator and each of these motor armatures. Each of the booster generators is supplied with a separately excited field winding 25b, 27b and 28b, the excitations of which determine the boosting or bucking effect of the booster generators. The booster generator field windings are energized by exciter generators 35, 37 and 38 having armatures 35a, 37a and 33a and separately excited field windings 35b, 37b and 370, and 38b and 38c.

The voltage of main generator 19, and thus the speed of the motors, is regulated by controlling the energization of main generator field winding by regulating the excitation of field winding 2% of exciter generator 29. Winding 29b is in turn controlled by the reversible direct current voltage output from output terminals 39a of saturable reactor or magnetic amplifier 39. This magnetic amplifier is supplied with alternating current at alternating current terminals 3% from a source such as an alternating current generator 139 and with regulating direct current at regulating terminals 39c, 39d and 39e.

Fig. 2 shows the arrangement of the cores, windings, rectifiers, resistors and transformer of amplifier 39. Referring to Fig. 2, magnetic amplifier 39 comprises a first pair of cores 39f having wound thereon reactance windings 39g each connected to be supplied from transformer 391 through rectifiers 39i with half wave rectified current to supply load resistor 39j with full wave rectified current, and a second pair of cores 39k having wound thereon reactance windings 39w each connected to be supplied from the transformer through rectifiers 39m with half wave rectified current to supply load resistors 39n with full wave rectified current. Load resistors 39] and 39;: are connected so that their voltage drops are opposed to provide a reversible direct current voltage output at terminals 39a. The transformer is connected to terminals 395. Terminals 390 are connected to saturating direct current signal windings 39o wound on each of the cores; terminals 39d are connected to saturating direct current reference windings 39p also wound on each of the cores; and terminals 39e are connected to saturating direct current compensating windings 39g also wound on each of the cores. The direct current windings are wound and energized so that on each core windings 39p and 394 aid each other and oppose windings 390.

Referring to Fig. l in conjunction with Fig. 2, terminals 39d are connected, through contacts 30b, across terminals 50a and contact arm 500 of motor operated rheostat 50. This rheostat is connected across bus 20. Motor 53,

25 having armature 53a and field winding 5312, may drive the movable contact arm 50c rotatably along resistor 50e, clockwise or counterclockwise, depending upon the direction of current through armature 53a, to difierent positions on resistor 50a Thus magnetic amplifier 39 is energized at reference terminals 1 by a speed reference voltage which appears across movable contact arm 50c and terminal 50a of the rheostat 50. Signal terminals 390 are connected through resistor 40 to terminals 91, 92, and are thus energized by a signal voltage which is a portion of the voltage of the main generator 19. Compensating terminals 39:2 are connected across the series field winding 16b of motor 1e to be energized by a compensating voltage which is the IR voltage drop across field winding 16b of motor 15.

The amplifier 39 balances, having zero voltage output at terminals 3% when on each of the cores the ampere turns of speed reference windings 39p plus the ampere turns of compensating windings 39: are equal to the ampere turns of signal windings 30o. Amplifier 39' thus differentially compares a portion or measure of the voltage of the main generator with a speed reference voltage and, in order to compensate for changes in motor load, modulates the generator voltage by the IR voltage drop of mill motor 16.

With main line switches 5, 6, 7 and 3 closed, the mill may be started by depressing push button 63 which completes a circuit from the positive side of direct current bus at terminal 100 through wire 101, wire 102, coil 30t of relay 30, and wire 103 to the negative side of, bus 20 at terminal 108. Armature 30s of the relay is thus picked up to the position shown. Relay coil 30! is kept energized and thus the armature is held in this position by the closing of hold in contacts 301' to complete a hold in circuit from the positive side of bus 20 at terminal 104 through wire 105, contacts 43, wire 106, contacts 301, wire 107, coil 30t, and wire 103 to negative terminal 108 on bus 20.

Depressing push button 63 also completes a circuit from positive terminal 100 through wire 101, wire 109, contacts 60, wire 110, through armature 53a of motor 53, wire 111, terminal 112 and resistor 113 to the negative terminal 114. This causes motor 53 to rotate arm 50c counterclockwise, increasing the voltage supplied to reference terminals 3% of amplifier 39. This will cause amplifier 39 to have an output in the direction to give exciter 29 an output that increases the voltage of the main generator thus raising the speed of the mill. Releasing push button 63 stops arm 50!: and amplifier 39 will balance when the portion of the generator voltage impressed on signal winding 390 is great enough to cause the ampere turns of that winding to equal those of windings 39p plus 39q. The voltage of the main generator is thus held at a voltage which runs the motors at a new speed corresponding to the new setting of arm 50c.

Depressing push button 64- completes a circuit from positive terminal 100 on bus 20 through resistor 115, wire 111, through armature 53a of motor 53, wire 116, contacts 44, wire 117, and wire 118 to negative terminal 114. This causes motor 53 to rotate arm 53a clockwise and, in a manner the reverse of that explained above, a new lower speed setting is obtained.

If the voltage of the main generator and thus the speed of the mill should increase above the voltage corresponding to the preselected speed, the energization of signal terminals 39c will increase in response to the increase in generator voltage. Amplifier 39 will then have an output causing eXciter 29 to decrease the voltage of the main generator to the value corresponding to the preselected. speed. If the voltage of the main generator and thus the speed of the mill should decrease, this speed variation is corrected in a manner the reverse of that just explained;

Thus, by control of the main generator voltage through rheostat 50, amplifier 39 and eXciter 29, the speed .of the mill can be raised, lowered, set and maintained at preselected desired values.

The current in the delivery tension motor 17 is regulated and maintained at preselected values through saturable reactors or magnetic amplifiers 57 and 47, exciter generator 37 and booster generator 27. The current in the delivery tension motor 18 may similarly be regulated and maintained constant through saturable reactors or magnetic amplifiers 58 and 48, exciter generator 38 and booster generator 28.

Fig. 3' shows magnetic amplifier 57 comprising a pair of cores 57,7 having wound thereon reactance windings 57g each supplied from transformer 57 t through rectifiers 557i with half wave rectified current and connected so as to supply full wave rectified current to output terminals 57a. The transformer is connected to alternating current terminals 57b. Regulating input terminals 570 are connected to regulating windings 57r wound on each. of the cores 57f. Regulating windings 571' are wound and connected so that the current flowing therethrough tends to saturate cores 57 and thus increases the direct current voltage output at terminals 57a. Magnetic amplifier 58 is identical to magnetic amplifier 57.

Fig. 4 shows magnetic amplifier 47 comprising a' first pair of cores 47 having wound thereon reactance windings 47g each connected to be supplied from transformer 47: through rectifiers 47 i with half wave rectified current to supply load resistor 47 with full wave rectified current, and a second pair of cores 47k having wound thereon-reactance windings 47w each connected to be supplied from the transformer through rectifiers 47m with half wave. rectified current to supply load resistor 47n with full wave rectified current. Load resistors 47 and 4711. are connected so that their voltage drops oppose each other to provide a reversible direct current voltage out put at terminals 47a. The transformer is connected to terminals 47 b. Regulating input terminals 470 are connected to regulating windings 47r wound on each of the cores 47 and 47k so that current flowing in one direction through windings 47r will tend to saturate one pair of cores and tend to desaturate the other pair to cause a voltage output of one polarity at terminals 47a, and that current flowing in the opposite direction through windings 47r will tend to saturate and desaturate the opposite pairs of cores to cause a voltage output of the opposite polarity at terminals 470. Magnetic amplifier 48 is identical to magnetic amplifier 47.

Referring to Fig. l in conjunction with Figs. 3 and 4', magnetic amplifier 57 has its direct current input regulating terminals 57c connected to control bus 20 through rheostat'70. The terminals 57b are connectedto a source of alternating current such as alternating current generator 157'. A load element such as reference resistor 77 is connected to the direct current output terminals 57a to provide a reference voltage between terminal 31 and an adjusting means such as'adjustable tap means 32.

No other resistor is in series with resistor 77 and thus 47. Amplifier 7 is supplied with alternating current at terminals 47b from a source such as alternating current generator 147. Amplifier 47 amplifies any control volt-, age resultingfrom this differential comparison providing a reversible direct current voltage output at terminals 47a which is'fed into field winding 37b of exciter 37 through contacts 30d.

This voltage output is proportional to the" control voltage and has a polarity dependent upon the polarity of the. control voltage. This voltage output energizes field winding 37b causing exciter 37 to excite the field of booster generator 27, causing the booster generator to aid or oppose the voltage of the main generator to bring the current in motor armature 17a to and maintain it at the preselected desired value called for by the value of the reference voltage selected by the position of adjustable tap means 32 and the setting of rheostat 70.

For example, for a given setting of rheostat 70 adjustable tap means 32 may be moved to the left on resistor 77 to a point corresponding to a desired value of current lower than the value of current presently in armature 17a. The reference voltage is now lower than the IR voltage drop across series field winding 17b and the difference between these voltages produces a control voltage of a first polarity to energize Windings 47r through terminals 470 to cause a voltage output of a first polarity at terminals 47a of amplifier 47. This voltage output energizes winding 37b, causing exciter generator 37 to excite booster generator 27 in a first direction causing the booster generator to oppose the voltage of the main generator to reduce the current in motor armature 17a. When the IR voltage drop across series field winding 17b equals the reference voltage, the regulating forcing effect stops, amplifier 47 balances, and the current equals the preselected desired value. Suppose now that the current decreases below the desired value. The IR voltage drop in the series field winding now will be less than the reference voltage and magnetic amplifier 47 will be oppositely energized by a control voltage of a polarity opposite the above described first polarity to produce at terminals 47a an output also of the opposite polarity than the above described first polarity to excite, through exciter generator 37, the booster generator in the direction opposite the above described first direction to cause the booster generator to aid the voltage of the main generator, increasing the current until the IR voltage drop across series field winding 17b again equals the reference voltage. The current has then been returned to the preselected desired value.

The current regulation for delivery tension motor 18 is accomplished in an identical manner as above explained but through magnetic amplifiers 58 and 48, exciter 38 and booster generator 28. Amplifier 58 compares to amplifier 57; amplifier 48 compares to amplifier 47; exciter 38 compares to exciter 37; and booster generator 28 compares to booster generator 27.

The desired division of load current between motors 17 and 18 is obtained by regulating rheostat 70. At midposition or its adjustable tap, amplifiers 57 and 58 receive equal energization. As the adjustable tap of rheostat 70 is moved from the midposition, the energization of one increases the other decreases. The setting of rheostat 70 thus selects and determines the ratio of the currents in motor armatures 17a, 18a.

Adjustable tap means 32 and 34 are ganged together by a linking means such as member 68 to increase or decrease simultaneously the value of currents to be maintained in motor armatures 17a, 18a. These adjustable tap means thus select and determine the value of the currents and maintain constant the above mentioned ratio of the currents.

The current in motor 15 is regulated and maintained at a preselected desired value by regulating the terminal voltage of motor 15 through booster generator 25, which aids or opposes the main generator voltage in response to the excitation of field winding 25b. Exciter generator 35 excites winding 25b. The exciter generator is in turn controlled by saturable reactor or magnetic amplifier 45.

Magnetic amplifier 45, referring to Fig. 5, comprises a first pair of cores 45 having wound thereon reactance windings 45g each connected to be supplied from transformer 45: through rectifiers 45i with half wave rectified current to suppiy load resistor 45 with full wave rectified current, and a second pair of cores 45k having wound thereon reactance windings 45w each connected to be supplied from the transformer through rectifiers 45m with half wave rectified current to supply load resistor 45n with full wave rectified current. Load resistors 45 and 45n are connected so that their voltage drops oppose each other to provide a reversible direct current voltage output at terminals 45a. Transformer 45! is connected to alternating current terminals 45b. Direct current input terminals 450 are connected to saturating direct current regulating Windings 45x, and direct current input terminals 45d are connected to saturating direct current windings 45x. Windings 45x and 452 are each wound on each of the cores 45 and 45k so that current flowing in one direction in either of the windings 45x, 45z will tend to saturate one pair of cores and desaturate the other pair to cause an output voltage of one polarity at terminals 47a, and that current flowing in either of the windings 45x, 452 in the opposite direction will tend to saturate and desaturate the opposite pairs of cores to cause an output voltage at terminals 45a of the opposite polarity.

Magnetic amplifier 55 is identical to magnetic amplifiers 47 and 48 and is constructed as shown for amplifier 47 in Fig. 4.

Referring to Fig. l in conjunction with Fig. 5, terminals 55b are connected to an alternating current source such as alternating current generator 155. Output terminals 55a of magnetic amplifier 55 have impressed thereacross a reference voltage dependent upon the voltage input to terminals 550 of the amplifier. The setting of rheostat85, which is across direct current control bus 20, determines the voltage applied to input terminals 55c and thus determines the reference voltage across terminals 55a. The IR voltage drop across series field winding 15b is connected to oppose this reference voltage and the difference between these voltages is a control voltage which is impressed across the input terminals 45d of amplifier 45. Amplifier 45, which is connected to an alternating current source such as alternating current generator 145, compares the reference voltage across terminals 55a, corresponding to the preselected desired value of current in motor armature 15a, with the IR voltage drop across field winding 15b, corresponding to the actual value of current in armature 15a, and amplifies any control voltage obtained to produce a direct current voltage output at output terminals 45a. The output voltage across terminals 45a is applied to field winding 35b of exciter 35, causing the exciter to energize Winding 25b of booster generator 25. Booster generator 25 thus brings the current in motor armature 15a to the preselected desired value. The current in motor armature 15a is thus regulated and maintained in a manner similar to that described hereinbefore for motors 17 and 18.

During threading of strip 12 through rolls 10 and 11 of the first and second stands of the mill, switch is closed to establish a speed matching control for maintaining the speed of mill motor 15 at the speed of mill motor 16. This control includes magnetic amplifiers 65, 66, and 45, and exciter generator 35. Magnetic amplifiers 65 and 66 are identical to magnetic amplifier 57 as shown in Fig. 3. Magnetic amplifier 45, shown in Fig. 5, has been heretofore described.

Resistor 96 is connected across terminals 61 and 62 including armature winding 16a and series field winding 16b of motor 16. Input terminals 660 of magnetic amplifier 66 are connected across the adjustable tap on resis tor 96 and a point between field winding 16b and armature winding 16a of motor 16 to form a bridge circuit. This bridge circuit responds to the difference between the terminal voltage across armature winding 16a and the IR voltage drop in field winding 16b. The IR voltage drop in field winding 16b is proportional to the IR voltage drop in armature winding 16a. The bridge circuit thus responds differentially to the terminal voltage of and the IR voltage drop in the armature winding to produce induced electromotive force of field winding 16!) is in a direction opposing the IR voltage drop across the armature winding, and therefore the value of the voltage appearing across terminals 660 will be higher than the value of the actual counterelectromotive force and proportional to an anticipated or greater counterelectromotive force which will exist at a future lower value of current to be reached by the decrease in the present current. If the current inthe circuit of armature winding 16a and field winding 16b increases from a present value, the

induced: electromotive' force of winding 16b aids the IR voltage drop, and therefore the voltage appearing across terminals 66c will be less than the value of the actual counterelectromotive force. The voltage across terminals 66c is thus proportional to an expected or anticipated lower value ofcounterelectromotive force which will exist ata future higher value of current.

- The bridge circuit above described produces a first voltage which is a measure of the counterelectromotive force of the armature winding of a motor and modulates that measurement in response to the rate of change of the current in the motor to produce a voltage proportional to the anticipated counterelectromotive force of the armature winding. Magnetic amplifier 66 thus responds to the anticipated counterelectromotive force of motor 16. Magnetic amplifier 65, in a like manner, responds to the anticipated counterelectromotive force of the armature winding 15a of motor 15. These amplifiers therefore respond very rapidly to. speed changes of their respective motors; Magnetic amplifiers 65 and 66 are connected at terminals 65b; 66b, to alternating current sources such as alternating current generators 165, 166. The amplifiers 65', 66 are identical to magnetic amplifiers 57 and 58 and are constructed as shown for magnetic amplifier 57 inFig, 3. The direct current output voltages of magnetic-amplifiers65 and 66, across resistors125 and 126, are differentially compared and fed into input terminals 450 of magnetic amplifier 45. Any difference in the output voltages, which are proportional tothe measured anticipated counterelectromotive forces of the armature windings of motors 15 and 16, reflects an anticipated difierence inthe motor speeds, and thus booster generator 25 is excited through exciter 35 and magnetic amplifier 45.. The booster generator aids or opposes the voltage of. the main generator to keep motor 15 at the same speed as. motor 16. Thus the. speeds of motors 15 and 16 are matched to facilitatethreading strip 12 through rolls l and 11. For example, if when threading a strip of material'th'rough the mill, the speed of motor 15 falls off due to. an increased load on rolls 10, the current through armature 15a will increase. During this increase of current, because of the measuring circuit above described, the. voltage across terminals 65c will be somewhat less than the actual counterelectromotive force of motor 15 and will be. proportional to' an anticipated counterelectromotive force of motor 15. The output of amplifier 65, in rapidresponse to the speed change, decreases to a value in the direction to cause exciter 35 to have an output through field winding 25b in a direction causing booster generator 25 to aid the voltage of main generator 19. This increases the speed and thus the counterelectromotive force of motor 15. When the speed of motor 15 is increased to be equal to the speed of motor 16, the opposing voltage outputs of amplifiers and 66 again balance. in this manner, the speed of motor 15 is maintained at the speed of motor 16 during the threading operation. When the strip has been threaded through the stands of the mill, switch is opened to remove the s eed. matching control.

Means are provided to stop and start the mill without opening the main line switches 5, 6, 7 and 8. Arm 5%, when rotated clockwise to its limit at or near one end of resistor 5%, will open contacts 43 and 44, and, when rotated counterclockwise to its limit at or near the opposite end of resistor 56c, will open contacts 60. Contacts 4-3 and 44 are normally held closed by spring 49. Contacts 66 are normally held closed by spring 59.

To stop the mill, push button 64 is depressed and a circuit is completed as heretofore described from terminal on the positive side of bus 20, through resistor 115,, through motor armature 53, through contacts 44 to negative terminal 114 on the bus 2%). Motor 53 thus rotates arm 50c clockwise and, with push button 64 held depressed, drives arm 500 to a point at or near one end of resistor 502 and element 50d opens contacts 43- and 44. Contacts 44, when opened, interrupt the circuit for energizing motor armature 53a through push button 64, and thus prevent damage to motor 53 by preventing the motor from working when stalled against spring 49. Contacts 43, when opened, interrupt the holding circuit for coil 30t andthus armature 30s drops out.

Contacts 3% open the circuit connecting amplifier terminals 39d and rheostat 56. This causes amplifier 39 to energizer exciter 29 such that it excites main generator 19, reducing the main generator voltage toward zero. The residual magnetism of main generator 19, and of booster generators 25, 27 and 28, however, causes these generators to develop some voltage output which drives the motors causing slow turning or creeping of the motors. In most systems this creeping is overcome by opening the main line switches, but this is unnecessary in this system, as will be hereinafter explained.

When contacts 3612 open, contacts 30a close to connect resistor 4-1 in parallel with resistor 40. This impresses across terminals 39c of amplifier 3? a second and greater portion of any existing generator voltage than the portion normally impressed thereacross solely through resistor 40. Or, stated another way, putting resistor 41 in parallel with resistor 40 by closing contact 36a effects an increase of the current in the signal winding circuit by decreasing the impedance of the resistive impedance in series therewith. This gives a suicide effect to the voltage of gen orator 19 by causing amplifier 39 to more forcefully energize exciter 29 so as to more forcefully excite generator 19 to rapidly reduce the generator voltage to zero.

The dropping out of armature 30s also opens contacts 36d to open and make ineffective field winding 37b of exciter. 37, preventing the booster generator from holding the current at a preselected operating value. Contacts 300 close to impress across field winding 37c any voltage existing across armature 17a of motor 17 causing exciter 37 to excite booster generator 27 to buck that voltage to zero. Field winding 370 is thereby substituted for field winding 37b and a suicide efiect is given to any voltage across armature 17a to quickly overcome the voltage developed by the generators because of their residual. magnetism and thus quickly stops motor 17.

Motor 18 is stopped in a manner identical to that described above tor motor 17 because contacts 30 open to 9 remove the effect of winding 38b of exciter 38 and contacts 30e close to energize winding 380.

Motor 15 is also stopped in response to the dropping out of armature 30s of relay 30. When contacts 30h open, the reference voltage across terminals 55a is removed from the circuit of input terminals 45d of amplifier 45. Contacts 30g simultaneously close to impress across input terminals 45d of amplifier 45 any voltage existing across field winding 15b. This causes amplifier 45 to energize exciter 35, which in turn excites the booster generator causing it to quickly reduce the current in armature 15a to zero, thus quickly stopping the motor.

The mill is thus stopped without opening any main line switches, by removing the normal regulating circuits and establishing suicide circuits to counteract voltage developed by the generators because of their residual magnetism.

The mill is started by depressing push button 63 which as heretofore described picks up armature 39s to reestablish the normal regulating circuits. The mill can be brought up to the desired speed by holding push button 63 depressed and releasing the push button when that speed is reached. Long continued depression of push button 63 rotates arm 500 to a point at or near the end of resistor 50c and element 50d opens limit contacts 60 to prevent the mill from exceeding an upper speed limit. The opening of contacts 60 also prevents motor 53 from being damaged by interrupting the energizing circuit of armature 53a through pushbutton 63 so that continued depression of push button 63 will not actuate the motor causing it to work when stalled against spring 59.

Features disclosed but not claimed herein are claimed in my copending applications, Serial Nos. 392,811 and 392,945, filed November 18, 1953.

It will be apparent to one skilled in the art that various changes and modifications may be made in the embodiments of this invention herein illustrated and described without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. The combination comprising a first circuit including in series the armature winding of a first motor, a field winding of said first motor and the armature winding of a first booster generator, a second circuit including in series the armature winding of a second motor, a field winding of said second motor and the armature winding of a second booster generator, a main generator for supplying voltage to said circuits, main generator control means including a main exciter generator and a main magnetic amplifier for controlling the excitation of said main generator, said main magnetic amplifier having a speed reference winding responsive to a speed reference voltage and a signal winding responsive to a first portion of the voltage of said main generator and adaptable to respond to a second portion of said voltage of said main generator, a first exciter generator for controlling the excitation of said first booster generator, a first magnetic amplifier for exciting said first exciter generator, said first magnetic amplifier having a control winding normally responsive to the difference of a first current reference voltage and the voltage drop in the series field winding of said first motor, said control winding being adaptable to respond solely to the voltage drop across the series field winding of said first motor, a second exciter generator for said second booster generator, a first field winding for said second exciter generator responsive to the diiference of a second current reference voltage and the voltage drop across the series field winding of said second motor, a second field winding for said second exciter generator adaptable to respond to the voltage across the armature Winding of said second motor, a rheostat having a positionable member for controlling the magnitude of said speed reference voltage to regulate the speed of said motors, and means for stopping said motors including switching means responsive to a limit position of said member for opening the circuit of said speed reference winding, for adapting said signal winding to respond to said second portion of the voltage of said main generator, for adapting said control winding to respond solely to the voltage drop across the series field winding of said first motor, for opening the circuit of said first field winding, and for adapting said second field winding to respond to the voltage across the armature winding of said second motor.

2. The combination comprising a circuit including in series the armature winding of a motor, a field winding of said motor and the armature winding of a booster generator, a main generator for supplying voltage to said circuit, main generator control means including a first exciter generator and a magnetic amplifier for controlling the excitation of said main generator, said magnetic amplifier having a speed reference winding responsive to a speed reference voltage and a signal winding normally responsive to a first portion of the voltage of said main generator and adaptable to respond to a second portion of said voltage of said main generator, a second exciter generator for said booster generator, a first field winding for said second exciter generator responsive to the difference of a current reference voltage and the voltage drop across said series field winding, a second field winding for said second exciter generator adaptable to respond to the voltage across the armature winding of said motor, a rheostat having a positionable member for controlling the magnitude of said speed reference voltage to regulate the speed of said motor, and means for stopping said motor including switching means responsive to a limit position of said member for opening the circuit of said speed reference winding, for adapting said signal winding to respond to a second portion of the voltage of said main generator, for opening the circuit of said first field winding, and for adapting said second field winding to respond to the voltage across the armature winding of said motor.

3. The combination comprising a circuit including in series the armature winding of a motor, a field winding of said motor and the armature winding of a booster generator, a main generator for supplying voltage to said circuit, main generator control means including a first exciter generator and a first magnetic amplifier for controlling the excitation of said main generator, said first magnetic amplifier having a speed reference winding responsive to a speed reference voltage and a signal winding normally responsive to a first portion of the voltage of said main generator and adaptable to respond to a second portion of the voltage of said main generator, booster generator control means including a second exciter generator and a second magnetic amplifier for controlling the excitation of said booster generator, said second magnetic amplifier having a control winding normally responsive to the difference of a current reference voltage and the voltage drop across said series field winding' and adaptable to respond solely to the voltage drop across said series field winding, a rheostat for controlling the magnitude of said speed reference voltage to regulate the speed of said motor, means for stopping said motor including switching means for opening the circuit of said speed reference Winding, for adapting said signal winding to respond to said second portion of the voltage of said main generator, and for adapting said control winding to respond solely to the voltage drop across said series field winding.

4. In combination, a circuit including in series the armature of a motor and the armature of a booster generator, a main generator for supplying voltage to said circuit, first means for normally controlling the excitation of said main generator, second means adaptable to control the excitation of said main generator, third means for normally controlling the excitation of said booster generator, fourth means adaptable to control the excitation of said booster generator, regulating means comprising positionable means for controlling said first means toregulate the speed of said motor, and means for stopping s'aid'motor comprising means actuated by saidpositionahle-me'ansafor adapting said second means to controlthe excitationof said'main generator and for adapting said fourth means to control the excitation of said booster generator.

5'. In'combination, agenerator, anexciter for said genera-tor, a magnetic amplifier for controlling said exciter, said magnetic amplifier having a reference winding responsive t'oa reference voltage and a signal Winding normally responsive to a first portion of the-voltage of'said generator and adaptable to respond to a second portion of the voltage of said generator, regulating means comprising positionable means for controlling the magnitudeof said reference voltage to regulate thevoltage of said generator and means for reducing the voltage of said generator to zero including switching means actuated by said positionable means for opening the circuit of said reference Winding and for adapting said signal winding to respond to said secondportion of the voltage of said generator.

6. The combination of agenerator, first means for normally controlling the excitation of said generator, second means adaptable to control the excitation of said generator, regulating means comprising positionable means for controlling said'first meansto control the voltage of said generator, and means for reducing said voltage to zero including means actuated by said-positionablem'eans for adapting said second means to control the excitation of said generator.

7. The combination comprising a generator, regulatingmeans for controlling said generator, including a first Winding connected to a first resistor responsive to a portionof the voltage of said generator, a second Winding responsive to a reference voltage for opposing said first the voltage of said generator causes said regulating means to decrease the voltage of said generator, and means for reducing said generator voltage to zero comprising means for deenergizing said second Winding and means for connecting a second resistor in parallel with said first:

resistor to increase the current due to said portion.

References Cited in the file of this patent UNITED STATES PATENTS winding, said first winding arranged that an increasein 

